1. Field of the Invention
This invention relates to an all-season pneumatic radial tire which can maintain steering stability and resistance to uneven wear on a dry road, a high performance of traction on a snow-covered road, and an improved resistance to wandering on a rainy road, throughout the whole period from the beginning to the end of its use.
2. Description of the Related Art
The provision of a light truck, particularly a sport utility vehicle (SUV) having a large body weight, has recently brought about a demand for the provision of all-season pneumatic tires having a high block rigidity as required for withstanding the weight of the vehicle body, and yet showing an excellent performance on a snow-covered road.
There is known a pneumatic radial tire having blocks defined by a plurality of circumferentially extending main grooves and a multiplicity of transverse grooves spaced apart from one another along the circumference of the tire. This type of tire usually has sipes formed in the blocks and extending along its width for achieving an improved traction on a snow-covered road, an improved resistance to wandering on a rainy road, a reduction of pattern noise made by the tread contacting the ground and an improved riding comfort.
There have been proposed various forms of sipes, and they can be classified mainly into OA and OB types by their cross sectional shape along their depth.
Examples of OA type sipes are shown at 801 and 802 in FIGS. 4 and 5. The sipes 801 and 802 made in two adjoining blocks 401 and 402, respectively, which are separated from each other by a circumferentially extending main groove 201, have portions 801a and 802a opening in the main groove 201 for connecting the sipes therewith, and having a horizontally straight cross section with a small depth above shelves 901a and 902a formed on the blocks 401 and 402. The sipes 801 and 802 have inwardly of the connecting portions 801a and 802a inner portions 801b and 802b cut with a greater depth and having a substantially semi-circular, or rectangular cross section. Tires having OA type sipes formed in the blocks are used mainly in summer, or throughout the year. FIG. 4 also shows a tread 101, circumferentially extending grooves 701 having a smaller open width than the main grooves 201, and transverse grooves 301. It also shows shoulder ends 501 and 502, and a tire centerline TC. It also shows by an arrow R the direction in which the circumference of the tire extends.
Examples of OB type sipes are shown in FIGS. 7(a0) and 7(b0), which correspond to FIGS. 4 and 5, respectively, and show a tire having the same tread pattern as shown in FIG. 4, but a different form of sipes. The sipes 811 and 812 are formed in two adjoining blocks 411 and 412, respectively, which are separated from each other by a circumferentially extending main groove 211. Neither of the blocks 411 and 412, however, has any shelf as shown at 901a or 902a in FIG. 5, and each sipe 811 or 812, therefore, does not have any connecting portion of reduced width as shown at 801a or 802a in FIG. 5, but is cut with a substantially equal depth from its end 811a or 812a opening in the main groove 211 through its inner portion 811b or 812b and has a rectangular cross section substantially like a parallelogram. Tires having OB type sipes formed in the blocks are used mainly in winter. FIG. 7(a0) also shows transverse grooves 311. It also shows by an arrow R the direction in which the circumference of the tire extends.
When a tire having OA type sipes is new, or in the beginning stage of its wear as shown in FIGS. 6(a0) and 6(b0), the sipes 801 and 802 having the cross sectional shapes as described make it possible to achieve steering stability and resistance to uneven wear on a dry road, a good performance of traction on a snow-covered road and resistance to wandering on a rainy road. As the wear of its tread surface proceeds, however, the connecting portions 801a and 802a of the sipes 801 and 802 have a reduced depth, and the shelves 901a and 902a are finally exposed on the tread surface with the loss of the sipes 801 and 802 as shown in FIG. 6(b1), resulting in an increase in rigidity of the blocks along the main groove 201 and a lowering of the traction on a snow-covered road and the resistance to wandering on a rainy road. Therefore, this type of tire is not necessarily satisfactory for use throughout the year, though it may be suitable in summer. FIGS. 6(a0) and 6(a1) show by an arrow R the direction in which the circumference of the tire extends.
On the other hand, a tire having OB type sipes 811 and 812 exhibits an improved traction on a snow-covered road, since the sipes 811 and 812 having the cross sectional shapes as described lower the rigidity of the blocks 411 and 412, but the lower rigidity of the blocks also brings about a lowering of steering stability on a dry road and the uneven wear, such as toe-and-heel wear, of the blocks 411 and 412 along their edges. Therefore, a tire having OB type sipes is not necessarily preferable for use throughout the year, though it may be suitable in winter. Moreover, it is too low in block rigidity for a tire used on a light truck, and particularly on a sport utility vehicle (SUV) having a large body weight. Therefore, the improvement in the block rigidity is required.
This invention has adopted a pneumatic radial tire having blocks formed on its tread by transverse grooves extending along its width and main grooves extending circumferentially across the transverse grooves, wherein every two of the blocks that are transversely spaced apart from each other by one of the main grooves have a sipe which extends transversely from one block to the other across the main groove and is open in the surface of the tread and the main groove throughout its length, the sipe having along its length and depth a cross section defined by an inner portion made in each block, a substantially U-shaped portion made in the bottom and side walls of the main groove, and a portion made in each block for connecting the inner and substantially U-shaped portions, the connecting portion having a depth reduced by a shelf formed on each block, the main grooves having a depth which is smaller than that of the transverse grooves, but which is equal to that of the transverse grooves where they cross the transverse grooves.
The tire of this invention, as described, differs from any known OA type tire mentioned in that its sipes have substantially U-shaped portions extending across the main grooves and remaining open therein for preventing any undesirable increase in rigidity of the blocks along the main grooves, even after the tire has got considerably worn, while the sipes always remain effective in forming edges on a snow-covered road. Thus, the tire of this invention exhibits an improved traction on a snow-covered road and an improved resistance to wandering on a rainy road over any known OA type tire mentioned.
The tire of this invention has a block rigidity which is not so low as that of any known OB type tire mentioned, but is comparable to that of any known OA type tire, since each of the blocks between which a sipe extends across a main groove has a shelf formed below the sipe portion of reduced depth connecting its inner and substantially U-shaped portions. Although the sipes extending between the adjoining blocks across the main grooves may lower the rigidity of the blocks along the main grooves, the circumferentially extending main grooves, which are smaller in depth than the transvserse grooves, except where the former cross the latter, make it possible to prevent any undesirably great lowering of block rigidity, while also restraining any unnecessary movement of the tire around its circumference. Thus, the tire of this invention maintains a satisfactorily high steering stability on a dry road, and resists any uneven toe-and-heel wear.
Therefore, the tire of this invention can effectively be used as even an all-season tire for a sport utility vehicle (SUV) having a large body weight without causing any reduction of steering stability on a dry road, or any toe-and-heel, or other uneven wear, while showing an improved traction on a snow-covered road and an improved resistance to wandering on a rainy road, from the beginning to end of its use.
It is, therefore, an object of this invention to provide an all-season pneumatic radial tire which exhibits an improved traction on a snow-covered road and an improved resistance to wandering on a rainy road, while not showing any undesirable reduction of steering stability on a dry road, or any uneven wear, from the beginning to the end of its use.